Developing a good ohmic contact to a semiconductor layer is critical for the operation, stability, and lifetime of the corresponding semiconductor device. Various approaches have been employed to form ohmic contacts. One approach, which can produce a good ohmic contact to a semiconductor layer, uses an annealing process. For example, titanium/aluminum (Ti/Al) is frequently used as an ohmic contact to n-doped nitride semiconductor layers. In this case, a titanium nitride (TiN) layer creates N vacancies in the underlying aluminum gallium nitride/gallium nitride (AlGaN/GaN) structure, which effectively dopes the material. Frequently, nickel (Ni) also is added to prevent diffusion and oxidation of the Ti/Al.
Another approach includes etching semiconductor layers and planting the ohmic contact into the etched cavity. For example, in one approach, recessed ohmic contacts are disclosed where a semiconductor device is formed by engineering a channel-forming layer grown on a semiconductor substrate with subsequent deposition of a Schottky layer. In this approach, the two dimensional electron gas (2DEG) is established at an interface between the Schottky layer and the channel-forming layer. Furthermore, in this approach, a gate electrode is formed on the Schottky layer via a cap layer and a recess-structured ohmic electrode is in ohmic contact with the 2DEG layer.
A similar technique has been used for Metal-Insulator-Semiconductor (MIS) high electron mobility transistors (HEMT). In this case, an insulating two nanometer thick AlN layer is removed and source and drain contacts are recessed. Contrary to the previous approach, the source and drain contacts are not recessed all the way to the 2DEG layer.
Recessed source and drain contacts also have been investigated in the context of transistor devices. Results have shown that a recessed source/drain structure can provide an ohmic contact with a much lower source/drain resistance than a conventional elevated source/drain contact. Furthermore, the recessed source/drain contact can reduce parasitic gate to source/drain capacitance over the conventional approach. A drawback of the recessed source and drain contacts is a presence of a short channel effect, which can deteriorate the device performance.
A recessed ohmic contact is useful as a way to access the 2DEG. A 2DEG is typically utilized in a HEMT, where the current path is formed at an interface between two types of semiconductor film having different band gaps. In order to support the 2DEG, the semiconductor layers typically comprise a channel-forming layer formed on a substrate and another layer forming a heterojunction with the channel-forming layer. For example, a GaN film can be used as the channel-forming layer, and an AlGaN film can be used as the layer forming the heterojunction with the channel-forming layer.
A recessed ohmic contact also is beneficial in cases when semiconductor layers do not support 2DEG, such as an ohmic contact formed for a light emitting device (LED). In this case, the recessed ohmic contact allows for a larger contact-to-semiconductor junction area, and as a result, a lower contact resistance.
Other approaches to reduce the ohmic contact resistance for AlGaN/GaN based HEMTs, for example, utilize a highly doped n+ cap GaN layer or selective implantation of silicon (Si) near the source and drain contacts.
Approaches for forming ohmic contacts are very different for n- and p-type contacts. For n-type contacts to n-type GaN, for example, the ohmic contacts are formed using a metal work function that is smaller than that of the n-type GaN based semiconductor. A frequently used metal is Ti, which has a work function, ϕm=4.33 eV. For Ti-based contacts to n-type GaN, which has a carrier concentration of 5 to 7×1018 cm−3, low contact resistances ranging from 10−5 to 10−8 Ωcm2 have been obtained.
Making a p-type contact to p-type GaN, for example, is much more difficult. In particular, it is difficult to grow well doped p-type GaN with a carrier concentration of more than 1018 cm−3 due to a high activation energy of acceptors. Additionally, it is difficult to find metals with a work function that corresponds to p-type GaN. Metals with a large work function, such as Ni, are typically used to form ohmic p-type contacts. The details of annealing are an important factor for contact performance. Various annealing approaches have been proposed, including annealing in air or oxygen to improve contact performance. Other approaches to improve the performance of an ohmic contact include various methods of treating a semiconductor surface. The possible methods include plasma and laser treatment. In addition, use of superlattices, strained semiconductor layers, and spontaneous polarization have been employed to achieve a high hole concentration and result in a low contact resistivity.